What does "hennaiatsu" actually mean? I don't know how it is written, but it was subtitled as "she's weird". For context: It was thought by one character after seeing another essentially tell others to go away so she could sleep in class. I could only find definitions for "hen" and "atsu". "Hen" could mean "strange", and "atsu" could be a shortening of "atsukamashi", which means "impudent"/"shameless". With "nai" in the middle, the closest I can come up with is "not just strange, but shameless", but I'm super iffy on that. Any help will be greatly appreciated!

I think you may be parsing what you heard incorrectly. From the context you've provided I would reckon what was said was , which in the context you provided means "She's weird," and more generally means "(a person) is weird." is a vulgar/informal/derogatory way of referring to any person or object.